In radiosurgery, very intense and precisely focused doses of radiation in a beam from a source outside a patient's body are delivered to a target region in the body, in order to destroy tumorous cells. Typically, the target region consists of a volume of tumorous tissue. Radiosurgery requires an extremely accurate spatial localization of the targeted tumors. Radiosurgery offers obvious advantages over conventional surgery, during which a surgeon's scalpel removes the tumor, by avoiding the common risks and problems associated with open surgery. These problems include invasiveness, high costs, the need for in-hospital stays and general anesthesia, and complications associated with post-operative recovery. When a cancerous tumor is located close to critical organs, nerves, or arteries, the risks of open surgery are even greater.
As a first step in performing radiosurgery, it is necessary to determine with great precision the location of tumors and any surrounding critical structures, relative to the reference frame of the treatment device. CT and MRI scans enable practitioners to precisely locate a tumor relative to skeletal landmarks or implanted fiducial markers. However, it is also necessary to control the position of the radiation source so that its beam can be precisely directed to the target tissue, with control of propagation in and through other body structures.
To effect such beam position control, stereotactic frames have been developed and used in the past for treatment of brain tumors. Stereotactic frames are rigid metal frames that are attached to the patient's skull and locked in place to provide a frame of reference for the surgeon during CT/MRI imaging, and for subsequent therapeutic treatment. A stereotactic frame is typically attached to the patient prior to scanning/imaging. The frame must remain in place while the surgeon is developing a computerized treatment plan, as well as during the actual treatment. During treatment, an x-ray or gamma ray source is precisely positioned with respect to the frame, so that the radiation can be administered according to the treatment plan.
While there are well-developed methods for attaching stereotactic frames to the skull for brain tumor treatment, attaching these frames to anatomical regions other than the skull in order to establish a stable frame of reference is too difficult to be practical. As one prior art example, a stereotactic frame that was deliberately constructed for the rest of the body (outside the head/neck region) required screws to be placed in the pelvis, incisions to be made along the spine to accommodate spinal clamps, and ten hours of general anesthesia to be administered to the patient while the frame was being attached to the patient, CT imaging performed, and radiosurgery undertaken. It is clearly not practical to perform such frame-based radiosurgery on areas other than the skull, and therefore the use of frame-based radiosurgery has so far been restricted to the treatment of intra-cranial tumors.
Despite the advantages of radiosurgery over open surgery, including significantly lower cost, less pain, fewer complications, no infection risk, no general anesthesia, and shorter hospital stays (most radiosurgical treatments are outpatient procedures), frame-based radiosurgery has a number of drawbacks. These drawbacks mostly relate to the use of the stereotactic frame. A stereotactic frame causes pain to the patient, since it has to be attached with screws. Also, a frame cannot be easily re-attached in precisely the same position for a subsequent radiation procedure, so that frame-based radiosurgical treatment is limited to smaller tumors (generally less than about three centimeters in diameter) that can be treated in a single procedure. Moreover, the frame must remain in place from the time of diagnostic CT and/or MRI scanning, through the entire period of treatment, which may extend over a multi-day period. Finally, the biggest drawback is that frame-based radiosurgery cannot be used for tumors located outside of the head and neck region, because of the above-described difficulty of attaching these frames to anatomical regions other than the skull. Frame-based radiosurgery therefore cannot be used to treat ninety percent of all solid tumors, because they occur outside of the head/neck region.
These drawbacks have lead to the development of a frameless stereotactic radiosurgery system, exemplified by the CyberKnife system (henceforth “CyberKnife”) made by Accuray, Inc., Sunnyvale, Calif. CyberKnife is an image guided robotic system which eliminates the need for the rigid stereotactic frames described above, and enables the treatment of extra-cranial tumor sites. CyberKnife provides numerous advantages compared to conventional stereotactic radiosurgery systems, including but not limited to: ability to treat tumors throughout the body, not just those located within the head/neck region; increased access to, and coverage of, any target volume; ability to treat tumors that are larger than about three centimeters in diameter; minimal constraints on patient set-up; ability to deliver a plurality of fractionated treatments; and enhanced ability to avoid damaging critical structures.
CyberKnife includes a robotic system onto which an x-ray linear accelerator (“linac”) is mounted, and a controller. The linac is adapted to selectively provide a precisely shaped and timed radiation beam. The controller uses CT and possibly MRI data, or other types of image data, that define the target tissue and important other bodily structures, together with treatment planning and delivery software to identify a series of landmarks within the treatment region, prior to surgery. CyberKnife may further include a stereo x-ray imaging system, which during treatment repeatedly measures the location and orientation of these landmarks relative to the linac. Prior to the delivery of radiation at each delivery site, the controller directs the robotic system to adjust the position and orientation of the linac in accordance with the measurements made by the x-ray imaging system, so that a desired series of radiation beams can be applied to the body, optimally dosing the target tissue while minimizing radiation to other body structures. In this way, CyberKnife allows accurate delivery of high doses of radiation, without requiring a stereotactic frame.
It is important to ensure that during the successive positionings of the linac during a treatment, the robotic system does not collide with objects (for example, parts of the patient's body, its own structure, or other equipment in the treatment room). Since patient setup is minimally constrained by a frameless radiosurgery system, it is difficult to have complete knowledge of the patient's body position when preparing a treatment plan, particularly regarding their arms and legs. An obstacle detection/collision avoidance system would therefore be desirable in frameless radiosurgery systems such as the CyberKnife.
Because of its ability to deliver fractionated treatments, CyberKnife is well adapted for radiotherapy, as well as for radiosurgery. The term radiotherapy refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy is typically about an order of magnitude smaller, as compared to the amount used in radiosurgery. Radiotherapy is frequently used to treat early stage, curable cancers. In addition to delivering radiation to cancerous tissue, radiotherapy systems generally also irradiate a certain amount of normal tissue surrounding the tumor. Typically, a series of relatively small doses are delivered over a number of days. Each radiation dose not only kills a little of the tumor, but also creates some collateral damage to healthy surrounding tissue, which however usually heals by itself, because it has a greater ability to repair, compared to cancerous tissue.
A collision avoidance system, referred to above, would also be desirable when CyberKnife is being used for radiotherapy, as well as for radiosurgery. For convenience, the term “radiosurgery” in this application shall henceforth mean “radiosurgery and/or radiotherapy.”